Depends on the country. Soviet ones (Gulags) were created for economic purposes as much as for political. The economic idea behind the Gulags was to extract free slave labor out of population and basically free calories out of their bodies - the prisoners were barely fed, so their bodies had to burn their own tissues (starting with fat) to survive. Millions of prisoners/slaves doing hard manual labor for no pay and eating 800 kalories per day helped the country's rapid industrialization in the 20s and 30s.